Molten aluminum and its alloys are very reactive, in particular with oxygen. Thus, a second phase, referred to as dross, is invariably present on the surface of aluminum melts. Depending on the nature of the raw materials used to generate the melt, and the environment within which the melt is generated, the dross may contain varying amounts of oxides, nitrides, carbides, entrapped metallic aluminum, metals and metal halides.
Aluminum drosses are classified according to the amount of NaCl/KCl salt they contain. Primary aluminum producers use little or no salt in their processes, and the dross they produce is referred to as "white dross". In the secondary refining of aluminum and in aluminum dross processing, NaCl/KCl salt is used more widely. The salts increase the interfacial tension of the dross/aluminum system, and allow the metallic aluminum to coalesce and be separated from the oxide more easily. The salt also helps protect the liquid aluminum from the atmosphere, thereby lessening the likelihood of further oxidation. The resultant drosses produced from these processes contain higher amounts of salt, and are referred to as "black dross".
Dross processors use great quantities of salt in their aluminum recovery process, and the non-metallic product of dross processing is referred to as salt slag; this material is typically composed of over 40% salt.
It is estimated that about 800,000 tons of all types of dross were produced in 1989. Dross is not currently classified as hazardous by the EPA, but black dross and salt slag could be so classified in the near future. Thus, processes are being developed to recycle or stabilize dross residue to produce environmentally harmless residues and technologies for processing aluminum dross to meet possible future environmental regulations concerning disposal of dross in landfills are being sought.
There are two very important aspects to be considered when processing aluminum dross. It is of extreme importance to maximize the recovery of metallic aluminum from the dross. It is of equal importance to produce a waste product that is environmentally compatible or salable to other industries.
The reactivity of aluminum, especially with oxygen, causes melt losses by formation of drosses in all casthouses, shelters, and foundries. The amount of dross produced is very different dependent on the type of operation; it is minimal for the simple holding of liquid aluminum in primary smelters, but large quantities can form when remelting scrap. In all cases, however, considerable quantities of nonoxidized aluminum are trapped in the dross, contributing to costly melt losses. Therefore, foundry engineers have developed many processes permitting the partial recovery of aluminum out of drosses.
Two general categories of dross processing have historically been employed--physical means and chemical means. In the past, recovery of aluminum was accomplished by rather simple physical techniques. For example, hot dross was taken from the furnace and spread on a concrete floor, allowed to cool and hand separated. In other techniques, the hot dross was cooled by placing it on a floor and letting it air cool, by placing it in a rotary tube and cooling it by spraying water onto the outer tube surface, or by pouring it onto a vibrating metal chute for more rapid air cooling. In all of these techniques, the cooled aluminum had then to be separated from the oxides formed during air cooling. All of these processes had serious environmental drawbacks. (R. Roberts Light Metal Age, 47, 6, (1989)).
Recently, improved methods for physical dross processing have been developed. The "aluminum recycling oxide separation"0 (AROS) method utilizes an enclosed, oxygen-starved environment for cooling, with collection of dust and fumes. (W. Franger, Light Metals 1987, Proc. TMS Conf. Light Metals, 116th Annual Meeting, Denver, Colo., 1987, 799.) The ALCAN process is conducted in an inert argon environment. (A. B. Innus, Light Metals 1986, Proc. TMS Conf. Light Metals, 115th Annual Meeting, New Orleans, La., 1986, 777.) ALCAN has developed another dross processing method to be used exclusively for no-salt drosses. (Second International Symposium: Recycling of Metals and Engineered Materials; van Linden, Stewart, Sahai, Eds.; TMS 1990, pp 451-462.) This process utilizes a rotary furnace, wherein the charge is heated by a single plasma-arc heater using nitrogen or air as the plasma-forming gas. The process is reportedly ineffective in producing environmentally benign end-products.
Several dross stirring processes to separate granular dross from molten aluminum have been developed, (O. Sivilotti, Light Metal Age, 1984, 42, 9.), and a compression method to squeeze the molten aluminum from the dross granules, at very high recovery rates, has also been developed (G. Zahorka, Light Metals, Proc. TMS Conf. Light Metals, 115th Annual Meeting, New Orleans, La., 1986, 769.)
Chemical methods of dross processing have also been developed. Disposal of the salt slag produced during the processing of aluminum dross and scrap is of great interest to the secondary aluminum industry. (M. J. Magyar et al., United States Bureau of Mines, Report 8446, 1980.) Melting of dross is often carried out under a salt flux cover to dissolve the contaminants, mostly aluminum oxide, and to optimize the recovery of aluminum metal. As the salt flux becomes contaminated with aluminum oxide, it is removed and customarily disposed of in a landfill. However, because the soluble salts in the flux are potentially polluting to surface and ground water supplies, this practice is being discouraged.
Two processes have been developed to deal with the salt slags produced in this way. The US Bureau of Mines (M. J. Magyar et al., United States Bureau of Mines, Report 8446, 1980), and Hudson and Olper (Engitec Impianti, S.p.A, unpublished report, 1990), have developed similar hydrometallurgical processes to deal with these types of waste.
Using these methods, the salt slag is crushed to produce slag fines. The fine material is sent to a leaching plant for salt brine production and the material is leached to obtain a salt brine with a concentration of approximately 25% by weight sodium/potassium chloride. This brine is filtered and condensed to produce a filtered alumina cake and a stream of salt crystals. Gases created during the process are passed through an afterburner.
Melting of aluminum by electricity, whether by resistance or induction furnaces, offers significant advantages over fossil-fuel melting with regard to energy efficiency and yield of high-quality product. Thus, it would be advantageous to develop more efficient means to melt aluminum for large-scale applications using electricity.
The economic and environmental incentives to recycle aluminum alloys, including dross, have resulted in considerable expansion of the secondary aluminum industry. Scrap for recycling can be of many forms, but often has a large surface area to volume ratio (examples being swarf, turnings, and lacquered scrap). This geometry, together with the high reactivity of aluminum alloys with oxygen, is the cause of metal loss problems associated with melting. Metal loss due to oxidation can be considerable because of the formation of dross. The magnitude of the problem is dependent on alloy composition, physical form of the charge and exposure time in a given environment. For aluminum alloys, the metal loss ranges from between 0.5 to 15% with 2% being considered typical, whereas for certain Al-Mg alloys typical values may be as high as 10%. Thus, would also be advantageous to develop electrical dross processing means which would allow recovery of aluminum from drosses.
Along with the heightened environmental concern regard the landfilling of potentially toxic aluminum drosses, costs of dross disposal have increased and are likely to increase further in the future. Dross is nominally composed of alumina (aluminum oxide) and metallic aluminum, with varying amounts of other oxides, nitrides, leachable metals and metal halides. Therefore any effective dross processing technology must be enough to accommodate compositional variation of significance and must also include a means of separating the metallic aluminum from the non-metallic phase in order to facilitate subsequent recycling.
The use of different fluxes at different temperatures allows for maximum flexibility in the processing of aluminum and salt slags. One means to attain such flexibility is indirect plasma-arc processing, as disclosed in this invention. In indirect plasma-arc processing, the arc is not transferred to the furnace charge electrically, as in many other processes. Thus, one example of plasma-arc prosessing is that in which electrodes arc against each other, with the plasma energy being transferred to the furnace charge primarily by convection and radiation. Another example of plasma-arc processing is that of a non-transferred plasma-arc torch, in which a plasma is generated within a water cooled torch between anode(s) and cathode(s) contained within the torch. In this case, energy is similarly transferred to the furnace charge by convection and radiation. Processing by both of these means, and other means of plasma-arc processing, is contemplated by this invention.
Depending upon the required end product, use can be made of indirect plasma-arc in combination with many available flux systems. If the end point of the processed dross is a landfill, then the cost of processing can be reduced by choosing the most cost effective, environmentally stable flux system. If the end product is a salable material, then the ingredients that will make up the flux need to be carefully selected depending on the desired market of the material.
Processing of aluminum drosses at high temperatures has the advantage of driving off volatile materials, such as chlorides, Cd, and Pb. This allows the resultant slag to be free of environmentally hazardous contaminants. The contaminants appear in the waste stream and are effectively isolated by selective condensation. Depending on the furnace atmosphere, high-temperature processing has the drawback of oxidizing, carburizing or nitrogenizing the entrapped aluminum, and reducing the recoverable fraction. This is addressed through a crushing and separation step conducted prior to high temperature processing, which allows a substantial separation of metallic aluminum from an alumina/metallic aluminum mixture, the latter forming part of the charging material.
The indirect plasma-arc heating process is thermally less efficient than direct heating, but is ideally suited for processing a nonconductive charge. The electrodes, either prepositioned or adjustable to arc toward another, can be directed toward the charge, making use of convective as well as radiative heat transfer, allowing optimal heating efficiency. The furnace capabilities are best utilized if the metallic aluminum content of the dross is low.
Both low and high temperature flux systems have been investigated in the present invention. The low-temperature fluxes were mainly borate based, while the higher melting temperature fluxes consisted of borosilicates, silicates, and calcium silicates. Both high and low-temperature processes have associated advantages and disadvantages. In all three of the base flux systems, compositions were identified that would dissolve aluminum dross. However, not all the slag systems could render the dross environmentally stable.
The advantages of low-temperature processing are in energy savings and increased aluminum recovery from the drosses. At lower temperatures, the amount of energy required to melt the flux-dross mixture is significantly reduced and there is no need for handling materials at elevated temperatures. For example, processing aluminum dross in a dc plasma-arc furnace at approximately 1200.degree. C. has the advantage of keeping the oxidation of the entrapped aluminum to a minimum.
One of the objectives leading to the process of this invention was to maximize the recovery of metallic aluminum from the dross. Lower operating temperatures lessen the thermodynamic driving force for the oxidation of aluminum. The only oxide-based flux system identified that would function at lower temperatures were borate-based fluxes. All the early flux compositions were based in this system. Borates melt at low temperatures and dissolve the drosses well and produce glassy, stable slags. However, they are extremely leachate sensitive and occasionally leachate toxic. Therefore, from the viewpoint of environmentally stable materials, borates cannot be employed for the processing of aluminum drosses.
Because borates cannot be used as the base flux system, elevated temperatures must be employed in order to meet the objective of producing an environmentally stable, landfillable, residue or slag. The use of elevated temperatures allowed for the investigation of several flux systems other than borates, these included silicates and calcium silicates. For example, Flux SL20 is presented in TABLE I along with other fluxes of this invention, although the invention is not limited to the fluxes listed therein. Flux SL20 is an example of a silicate-based flux, modified by equal amounts of CaO and MgO, on a weight basis, and can dissolve up to 30 wt % Al.sub.2 O.sub.3 at 1550.degree. C.
Raw materials are inexpensive and the flux systems chosen have exhibited high solubility for alumina (Al.sub.2 O.sub.3). As indicated by the leachate tests, once the dross has reacted with the basic flux, such as that presented in TABLE I, it is rendered inert and environmentally stable. For example, SL21 has a solubility for alumina of 35 wt % at 1400.degree. C. The resultant slag after processing will consist of CaO--Al.sub.2 O.sub.3 --S.sub.i O.sub.2 --MgO. (See Table II.)
The process described in this invention is by no means limited to processing dross with only a single flux composition. Depending on the desired product and base dross, an oxide flux can be engineered to produce the required product upon reaction with the dross. For some applications, the product may not be required to be environmentally benign. For example, borates have been demonstrated to be leachate toxic; however, borates can be used if the required reaction product was a feed stock for borax-based flux manufacturers. Because the processing temperatures will be elevated, the crushing of the dross to liberate maximum entrapped aluminum may still be desirable prior to melting.
Thus, a waste stream processor coupled to an indirect-arc furnace employing novel and innovative oxide flux technologies has been developed in response to the aforementioned environmental and economic considerations. A dross charge is melted in the presence of a basic flux by one or more arc-forming electrodes with a gas, provided through the electrode to form a plasma medium.
The products formed by melting the dross in this manner are removed as molten oxides, for resale or as environmentally benign end-products, or both. The volatile products may be removed by an off-gas duct or similar mechanism.
The present invention involves novel fluxing technology that will render the by-products of aluminum dross either environmentally stable or will compositionally modify the dross to make it attractive to other industries as a raw material.
The processor has been developed to handle all manner of aluminum production waste streams and render them environmentally compatible or alter them into salable products. The process of this invention is more clearly described in the following sections of this application.
It is an object of the present invention to provide a dross treatment process which maximizes the recovery of metallic aluminum while emphasizing the production and treatment of waste products to result in end-products that are environmentally benign, or salable to other industries, or both.
It is a further object of this invention to provide a dross treatment process capable of flexibly responding to the treatment of drosses of varying composition, by providing flexible flux engineering to obtain desirable end-products.